Conventionally, an angle of rotation of a polarization plane rotated by a sample, that is, an optical rotation angle (also referred to as “an optical rotary power”) is calculated from a signal obtained by causing a linearly polarized light to be incident on the sample, causing a light-flux transmitted by the sample to be incident on an analyzer, and photoelectrically converting the light-flux into an electric signal using a photodiode.
Assuming that a gradient of a transmission axis of the analyzer with respect to a transmission axis of a polarizer is θ and that an optical rotation angle by the sample is α, an intensity I of the light received by the photodiode is represented by the following Equation (1): I=T×I0 cos(θ−α)2. In the Equation (1), symbol T denotes transmissivity in consideration of all attenuations caused by reflections and absorptions of the sample, the polarizer, and the analyzer, and symbol I0 denotes an intensity of the incident light.
As evident from the Equation (1), a minimum point is obtained per rotation angle π (rad) following rotation of the analyzer. The optical rotation angle can be calculated from the angle of the analyzer at this minimum point.
For purposes of improving accuracy and sensitivity, a method for vibrating the polarization plane is normally used, which method will be explained with reference to FIG. 18. FIG. 18 is an explanatory diagram of a conventional optical rotation angle measuring apparatus using a method for vibrating the polarization plane. A monochrome light emitted from a light source 1821 is incident on a polarizer 1822 vibrated at a frequency f and an angular amplitude θ by a polarizer driver circuit 1829. The incident light is thereby transformed into a linearly polarized light having a rotated and vibrated polarization plane.
If a light-flux of this linearly polarized light is made incident on a sample 1825 and transmitted by an analyzer 1823, a signal at the frequency f is obtained from a photodiode 1824. If the polarization plane is rotated by α by the optical rotary power of the sample 1825, the signal having a phase inverted according to whether the sample 1825 rotates clockwise or counterclockwise can be obtained by arranging the polarizer 1822 and the analyzer 1823 orthogonal to each other.
The signal obtained from the photodiode 1824 is amplified by an amplifier circuit 1826, and synchronized and rectified by a rectifying and filtering circuit 1827, thereby calculating the phase of the signal. According to the phase, the analyzer 1823 is rotated forward or backward through an analyzer driver circuit 1828. As a consequence, the angle of the analyzer is determined by optical null method so that a quantity of transmitted light is a minimum. The angle of the analyzer at this equilibrium point corresponds to the optical rotation angle by the sample 1825.
Alternatively, if the phase of the signal obtained from the photodiode 1824 is detected at the modulation frequency f, a signal component at the frequency f is extracted, and the angle of the analyzer is adjusted so that an intensity of this signal is a minimum, the same result can be obtained.
As the method for vibrating and rotating the polarization plane, a method using a Faraday rotor that utilizes Faraday effect is also known as well as the method for mechanically rotating the polarizer. For example, a method for measuring the optical rotation angle by applying a magnetic field and using the Faraday effect has been suggested (see, for example, patent document 1: Japanese Patent Application Laid-Open No. H9-145605 (FIG. 7)).
As a similar method for measuring the optical rotation angle, a method using a liquid crystal element as a modulation element represented by the Faraday cell explained above has been suggested. This method has advantages of low power consumption driving, size reduction, and the like (see, for example, patent document 2: Japanese Patent Application Laid-Open No. 2002-277387 (FIG. 7)).
As for use of the liquid crystal element to rotate a linear light, there is known a Senarmont azimuth rotator configured by a combination of a liquid crystal element and a quarter-wave plate. As a development of this rotator, there is known an invention of a device that is configured so that three liquid crystal elements to which a variable voltage can be applied are arranged in series in a light irradiation direction and that enables optical modulation having higher flexibility (see, for example, patent document 3: Japanese Patent Application Laid-Open No. H7-218889 (FIG. 3)).
As a concentration measuring apparatus using an optical rotary power of a liquid crystal element, there is known an invention characterized by not including a conventional mechanical operating unit (see, for example, patent document 4: Japanese Patent Application Laid-Open No. 2001-356089 (FIG. 2)).
As a further development, there is known an invention that enables high accuracy, stable measurement by periodic phase modulation using a liquid crystal element (see, for example, patent document 5: Japanese Patent Application Laid-open No. 2002-277387 (FIG. 3)). FIG. 19 is an explanatory diagram of an optical system of the conventional concentration measuring apparatus.
A light-flux emitted from a laser diode 1921 is collimated into a parallel light by a lens 1922. This parallel light is polarized by a polarizer 1923A into an linearly polarized light vibrating in a direction inclined by 45 degrees with respect to a vertical direction.
Both of or one of polarized components of the light emitted from the polarizer 1923A in horizontal and vertical directions is subjected to phase modulation by a liquid crystal element 1931. The liquid crystal element 1931 is a homogeneous alignment-type liquid crystal element in which major axes of liquid crystal molecules are aligned in either the horizontal direction or the vertical direction. In this homogeneous alignment-type liquid crystal element 1931, the liquid crystal molecules stand upright when a voltage is applied thereto, and a refractive index in a molecular major axis direction is changed, whereby phase modulation can be performed. If the liquid crystal element 1931 modulates the phase of only one polarized component, linearly polarized lights interfere with each other.
The transmission light transmitted by the liquid crystal element 1931 is split into a reflective light and a rectilinear light by a half mirror 1924. The rectilinear light is incident on a quarter-wave plate 1926A with a horizontal axis and a vertical axis inclined by 45 degrees. As a result, vibrating components of the incident rectilinear light in the horizontal and vertical directions can be converted into circularly polarized components rotating in opposite directions.
The rectilinear light transmitted by the quarter-wave plate 1926A is made incident on a sample 1925, whereby a phase difference of ±θ is generated between a clockwise circularly polarized light and a counterclockwise circularly polarized light according to the optical rotation angle by the sample 1925. Namely, if the rectilinear light transmitted by the quarter-wave plate 1926A is incident on the sample 1925, the clockwise circularly polarized light and the counterclockwise circularly polarized light are emitted from the sample 1925 with the phase difference of ±θ therebetween.
On the other hand, the reflective light from the half mirror 1924 is incident on a polarizer 1923C. The light transmitted by the polarizer 1923C is incident on a photodiode 1929B, and converted into an electric signal by a photodiode 1929B, thereby generating a beat signal.
The clockwise circularly polarized light and the counterclockwise circularly polarized light emitted from the sample 1925 are transmitted by a quarter-wave plate 1926B having an optical axis equal to or orthogonal to that of the quarter-wave plate 1926A. The clockwise circularly polarized light and the counterclockwise circularly polarized light are thereby converted into polarized components orthogonal to the horizontal or vertical direction, respectively.
The transmission light transmitted by the quarter-wave plate 1926B is transmitted by a polarizer 1923B inclined by 45 degrees with respect to the horizontal or vertical direction. Interfering signals between the linearly polarized lights can be thereby obtained. Furthermore, since the light-flux of one of the linearly polarized lights is phase-modulated, a beat signal is obtained and converted into an electric signal by a photodiode 1929A. The beat signal obtained by the photodiode 1929B is not influenced by the optical rotation angle by the sample 1925, so that the optical rotation angle by the sample 1925 can be obtained based on the phase difference between the signals from the photodiodes 1929A and 1929B.
However, as explained, it is necessary to mechanically rotate the polarizers and perform azimuth rotation and modulation using the Faraday effect as represented by the Faraday rotor so as to realize an azimuth rotator necessary for the measurement. This disadvantageously makes the optical rotation angle measuring apparatus large in size and expensive.
Meanwhile, if the liquid crystal element is used, the optical rotation angle measuring apparatus can be made small in size and driven at low power consumption. However, the optical rotation angle measuring apparatus using the liquid crystal element is disadvantageous in large fluctuations due to external environment such as temperature or atmospheric pressure. As a result, it is necessary to additionally provide a device such as a temperature controller in the optical rotation angle measuring apparatus so as to improve stability of measurement results. Such an apparatus is similarly disadvantageously made large in size, and expensive.
Furthermore, if an optical measurement system using the liquid crystal element is to be specifically realized, it is important to determine what structure is to be adopted to hold the liquid crystal element as well as optical components. However, the conventional techniques do not at all disclose this respect, so that the conventional techniques have a disadvantage in that stable and precise measurement cannot be ensured.
The present invention has been achieved in view of the conventional disadvantages. It is an object of the present invention to provide an optical rotation angle measuring apparatus that can be made small in size and ensure high accuracy in measuring the optical rotation angle with a simple configuration.